Former Irish president Mary Robinson, screen legend Sidney Poitier and Bangladeshi economist and Nobel peace laureate Muhammad Yunus were also honored with the Presidential Medal of Freedom in a ceremony in the White House.

"No barriers of race, gender or physical infirmity can restrain the human spirit, and that the truest test of a person's life is what we do for one another," said Obama, as he introduced the 2009 honorees.

"There are many honors and privileges bestowed on the occupants of this house. But few mean as much to me as the chance to award America's highest civilian medal to the recipients that are here today."

Before placing the blue-ribboned medal around the neck of wheelchair-bound scientist Hawking, Obama joked he was a "brilliant man and a mediocre student."

"From his wheelchair, he's led us on a journey to the farthest and strangest reaches of the cosmos. In so doing, he has stirred our imagination and shown us the power of the human spirit here on Earth."

Obama lauded Tutu for preaching amid tear gas and police dogs during the horror of apartheid South Africa.

"Later, when a free South Africa needed a heart big enough to forgive its sins, Archbishop Desmond Tutu was called to serve once more," Obama said, praising him as a "voice of the oppressed, cantor of our conscience."

Obama's selection of Robinson to receive the medal sparked anger among some pro-Israel lobby groups, who accused her of bias against the Jewish state.

But Obama described the former United Nations Commissioner for Human Rights as an "advocate for the hungry and the hunted," though his spokesman Robert Gibbs earlier said the president did not agree with all of her beliefs.

"It's been said that Sidney Poitier does not make movies, he makes milestones -- milestones of artistic excellence, milestones of America's progress," Obama said of the famed star of "Guess Who's Coming to Dinner."

Poitier, the president added, had not only entertained, but shifted racial attitudes, broadened hearts and brought Americans together.

Poignantly, another medal recipient, Senator Edward Kennedy, was missing from the ceremony, and was represented by his daughter Kara.

Kennedy, who is battling brain cancer, suffered another blow on Tuesday with the death of his sister Eunice Kennedy Shriver.

"The life of Senator Edward M. Kennedy has made a difference for us all," Obama said.

Obama also honored economist Yunus, who lifted millions of people out of poverty by offering them small loans.

"Muhammad Yunus was just trying to help a village, but he somehow managed to change the world," he said.